You could show (at the top of the autosuggest) that no results match and then a separator, and then give some partial matches or the most popular choices. That way, they see no results were found but might see what they need anyway.

From a usability standpoint, autocomplete should really be using terms that exist, that will return results. Not sure why a user would want a field to autocomplete only to be told that there are zero results for that term. It's not very elegant.

Plus, as I fill in a field form and see terms suggested to shorten my typing and time, I am probably anticipating that those terms will give me what I want in the way of search results.